Semiconductor electronic devices comprise an electronic circuit formed on a small plate of semiconductor material, known as a die, having a surface area of a few square millimeters. The electronic circuit is typically monolithically integrated with the die. The die is then enclosed in a case or package for heat and mechanical protection. Plastic packages will be specifically considered herein.
These type devices require suitable support and electrical interconnection arrangements for their electrical connection to an external circuit. For this purpose, a lead frame is cut from a thin metal plate to include a plurality of narrow strips forming electrical connectors or leads having one end connected electrically to the integrated circuit by conductive wires. The wires are soldered between the leads and corresponding terminations within the integrated circuit. The other or free ends of the leads extend from the plastic package to provide electrical connection pins for the device.
Since certain new generation devices may require over 100 leads for as many pins, the metal strips have to be made very thin and, therefore, are easily distorted. To prevent this problem, a dam bar is provided to link the frame leads together.
The power integrated circuits discussed below are commonly referred to as power packages, and are likely to release relatively large amounts of heat. A power package denotes a device that is adapted to dissipate heat. In this case, the die would be thermally coupled to a heat sink element for transferring the heat generated during its operation to the outside environment.
The heat sink element is made of metal, or another good heat conductor, in the form of a cylinder or a flattened parallelepiped having a much larger mass than the electronic device. As is typical in a power package, the heat sink element has only one of its major surfaces exposed. The other major surface and all the lateral surfaces of the heat sink element are isolated since they are covered under the material from which the plastic package is formed.
In some devices, the heat sink element has at least one of its sides laterally exposed from the plastic package. FIG. 1 is a bottom view of an electronic device 1 after the plastic package 2 has been molded around it. The plastic package 2 encapsulates a heat sink element 3 while leaving only a major bottom surface 4 of the latter exposed. This surface is flush with the underside of the plastic package.
A frame 5 is mounted on the opposite surface of the heat sink element 3 from the surface 4, and is affixed thereto by riveting or soldering. The frame 5 is partly embedded in the plastic package 2 such that the ends of the leads 6 externally project from the plastic package 2. A dam bar 8 is provided to link the leads 6 together.
In this prior art device, the heat sink element 3 has two extensions 7, the ends of which project out through opposite sides of the plastic package 2. In particular, the extensions 7 in this type of device are centrally located on two opposed sides of the frame 5. Leads 6 are not formed corresponding to the extensions 7.
The standard process steps for molding a conventional plastic package with an exposed heat sink will now be reviewed. Integrated circuits, complete with their heat sinks, are introduced into a mold having cavities aligned with the individual devices for molding their respective plastic package. Each cavity is injected with a molten electrically insulative material at a high temperature. This material is typically a synthetic resin, e.g., an epoxy resin, for forming the plastic package.
To arrange for the heat sink element 3 to have an exposed surface, the mold is formed at the peripheral portions or extensions with cavities, in which plastic blocks or tips 7a are molded. These tips 7a will abut the exposed portions of the extensions 7 in the finished package.
The molding process itself comprises injecting the resin into the cavity through a gate 9, as best shown in FIG. 2, to form the plastic package and the plastic tips 7a. For the device shown in FIG. 2, the injection gate 9 would be provided in the mold at one corner of the device.
The molding process continues through steps to be carried out within the mold, namely melting the plastic material, expanding the melt inside the mold, and solidifying the melt. After an initial cooling step, followed by thermal curing processes to thoroughly polymerize the resin, the plastic package is completely formed. The series of plastic packages are ready for removal from the mold.
The plastic tips 7a are then removed from the finished plastic packages to uncover the portions of the extensions 7 that are to be exposed. Although advantageous in several aspects, this method has several drawbacks. New power devices require a larger number of pins for connection to the outside environment. Thus, the lateral regions of the plastic packages must be left unobstructed, such that the leads can be provided all around the peripheral surface of the package.